Fluid catalytic cracking (FCC) processes are commonly used in refineries as a method for converting feedstocks, without requiring additional hydrogen, to produce lower boiling fractions suitable for use as fuels. While FCC processes can be effective for converting a majority of a typical input feed, under conventional operating conditions at least a portion of the resulting products can correspond to a fraction that exits the process as a “bottoms” fraction. This bottoms fraction can typically be a high boiling range fraction, such as a ˜650° F.+ (˜343° C.+) fraction. Because this bottoms fraction may also contain FCC catalyst fines, this fraction can sometimes be referred to as a catalytic slurry oil.
Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace wherein the feedstock, typically comprising crude or a fraction thereof optionally desalted, is heated sufficiently to cause thermal decomposition of the larger molecules. Among the valuable and desirable products include light olefins such as ethylene, propylene, and butylenes. The pyrolysis process, however, also produces molecules that tend to combine to form high molecular weight materials known as steam cracked tar or steam cracker tar, hereinafter referred to as “SCT”. These are among the least valuable products obtained from the effluent of a pyrolysis furnace. In general, feedstocks containing higher boiling materials (“heavy feeds”) tend to produce greater quantities of SCT. It should be noted that the terms thermal pyrolysis unit, pyrolysis unit, and steam cracker are used synonymously herein; all refer to what is conventionally known as a steam cracker, even though steam is optional.
Steam cracking processes are commonly used in refineries as a method for producing olefins from heavy oils or other low value fractions. A side product generated during steam cracking can be steam cracker tar. Steam cracker tar can typically be a highly aromatic product with a boiling range similar to a vacuum gas oil and/or a vacuum resid fraction. Conventionally, steam cracker tar can be difficult to process using a fixed bed reactor because various molecules within a steam cracker tar feed are highly reactive, leading to fouling and operability issues. Such processing difficulties can be further complicated, for example, by the high viscosity of the feed, the presence of coke fines within a steam cracker tar feed, and/or other properties related to the composition of steam cracker tar.
U.S. Publication 2017/0002279 describes methods for fixed bed hydroprocessing of catalytic slurry oil under various conditions.